1. Field of the Invention
The present invention relates to a method and apparatus to modulate sub-channel pixels in a multi grayscale monochrome output apparatus, and more particularly, to a method and apparatus to modulate sub-channel pixels in a multi grayscale monochrome output apparatus that can represent a high grayscale input image on a low grayscale output apparatus through sub-channel pixel modulation.
2. Description of the Related Art
In order to represent medical images, such as MRI or CT, a monochrome display is required. The monochrome display includes a monochrome display that represents information of one pixel of an image in the display as one pixel, and a monochrome display that represents the one pixel information using sub-channel pixels of R, G, and B with no color filters. In the latter case, the color filters are removed from a color display, and thus a manufacturing process is simple. However, since the number of cases of the one pixel information of the input image to be represented by the sub-channel pixels of R, G, and B with no color filters is large, an appropriate pixel representation method is required.
FIG. 1 shows a change of each sub-channel pixel in a monochrome output apparatus. For example, in the monochrome output apparatus, in order to represent “0”, all the sub-channel pixels become “0”. Further, in order to represent “1”, all the pixels of the first, second, and third channels become “1”. Information between “0” and “1” can be represented by combining the individual sub-channels. Accordingly, excess grayscale levels can be represented by varying pixel information of the sub-channels.
Meanwhile, in a case when a high grayscale input image is represented in a low grayscale output apparatus, when a required color cannot be used, a dithering method that represent a similar color by mixing dots of different grayscale colors may be primarily used. For example, gray having a specific grayscale level may be represented for the entire image according to a ratio of black dots and white dots within a certain plane in a display or a printing apparatus, or pink having various grayscale levels may be represented according to a ratio of red dots and white dots. The dithering method is primarily used to increase reality of an image and allows a rough and uneven outline so as not to meet a person's eye in a low definition output apparatus.
Known technologies using dithering include a random dithering method that adds a random signal generated from a random generator to the least significant bit (LSB) of an input image signal and a screen dithering method that prescribes a mask value for dithering and then adds the mask value to the least significant bit or compares the mask value with the least significant bit so as to determine a weighted value.
In the random dithering method, arithmetic is simple, but an ununiform pattern may occur since noise is added to an input value. Meanwhile, in the screen dithering, since the prescribed mask pattern is added to or compared with the input value, a regular lattice pattern occurs.
As such, when the known method is used, the entire grayscale level is improved, but there is a problem in that deterioration of image quality in an image subjected to dithering, such as stain or regular lattice pattern, occurs.
FIG. 2 shows an example where screen dithering according to the related art is performed.
As shown in FIG. 2, when 10-bit input signals are input for four pixels but 8-bit output signals are output, the input signals cannot be completely represented. For example, referring to FIG. 2, when it is assumed that the 10-bit pixel information is 494, 488, 491, and 485, the 8-bit output information is theoretically 123.50, 122.00, 122.75, and 121.25, respectively, but a part of the 10-bit input signal may be lost since the 8-bit information is obtained by truncating or reducing two bits among the ten bits. For example, when three sub-channels having a low grayscale output value exist, high grayscale input information can be represented by modulating pixel information of the first, second, and third channels.
For example, in the screen dithering method of FIG. 2, the lowest two bits are set as the least significant bits (LSB) of the input image signal. In the invention, the matrix having the least significant bits of each pixel in a pixel region subject to modulation is defined as original patterns of first, second, and third channels. For example, when screen dithering is applied to four pixels, the original patterns of the first, second, and third channels having the two least significant bits become the 2×2 matrixes. When the underlined two least significant bits from the information of each of four pixels shown in FIG. 1 are selected and the selected bits are represented in an order of (1, 1), (1, 2), (2, 1), and (2, 2), a 2×2 matrix of (2, 0, 3, 1) is obtained. In general, binary arithmetic is performed, and the above matrix is represented by (10(2), 00(2), 11(2), 01(2)) in binary. In the specification, however, decimal arithmetic is performed. In addition, in the detailed description of the invention, the 2×2 matrix is basically represented in an order of (1, 1), (1, 2), (2, 1), and (2, 2).
As described above, a single input signal having an initial input value of 10 bits is divided in to the first, second, and third sub-channels. Accordingly, by selecting the individual two least significant bits, the 2×2 matrixes serving as the original patterns of the first, second, and third channels respectively become the 2×2 matrixes of (2, 0, 3, 1).
In case of screen dithering, dithering can be performed by applying a prescribed mask pattern. For example, the first channel mask pattern becomes a 2×2 matrix of (0.5, 1.5, 2.5, 3.5) in an order of (1, 1), (1, 2), (2, 1), and (2, 2). Similarly, the second channel mask pattern becomes a 2×2 matrix of (3.5, 2.5, 0.5, 1.5), and the third channel mask pattern becomes a 2×2 matrix of (1.5, 2.5, 3.5, 0.5). The values of the mask patterns for screen dithering can be arbitrarily defined, and thus values different from the above-described values may be used as the values of the mask patterns.
Then, the first channel original pattern and the first channel mask pattern are compared with each other. As the comparison result, when the value of the original pattern is larger than the value of the mask pattern, a weighted value “+1” is determined and, when the value of the original pattern is smaller than the value of the mask pattern, a weighted value “0” is determined. Accordingly, from the comparison result of the original pattern and the mask pattern, a 2×2 weighted pattern of (+1, 0, +1, 0) is obtained.
Similarly, when second mask pattern and third mask pattern are applied to the second channel and the third channel, respectively, a weighted pattern of the second channel becomes a 2×2 matrix of (0, 0, +1, 0), and a weighted pattern of the third channel becomes a 2×2 matrix of (+1, 0, 0, +1).
Accordingly, the pixel values of each sub-channel can be modulated by adding the 2×2 matrix serving as the weighted pattern of each channel to the higher-order 8 bits. For example, as shown in FIG. 2, for the information of the four pixels as the input signal, the weighted values of each channel are added to the higher-order 8 bits among the original 10 bits, and thus the final pixel values of each channel are determined. As for the first channel among the sub-channels, since the values of the higher-order 8 bits from the input values of the four pixels are (123, 122, 122, 121), and the weighted values are (+1, 0, +1, 0), (124, 122, 123, 121) is obtained by adding the two 2×2 matrixes.
Similarly, if screen dithering is applied to the second channel and the third channel, the 8-bit output values of the second channel become (123, 122, 123, 121), and the 8-bit output values of the third channel become (124, 122, 122, 122).
Accordingly, when the 10-bit input values are given for the four pixels and the 8-bit output values are output, the average values of the output values of the four pixels in the first, second, and third sub-channels are (123.66, 122, 122.66, 121.33). Therefore, it is possible to approximate the 10-bit input values to theoretical 8-bit values (123.50, 122, 122.75, 121.25).
That is, even though the 8-bit values are output as output information, the 10-bit input values can be approximately represented by averaging the 8-bit output values of the first, second, and third sub-channels. Specifically, when high grayscale input information is output at a low grayscale level, grayscale characteristics can be improved by processing the output information of a plurality of sub-channels having low grayscale levels using the mask. Therefore, the loss of the information due to low grayscale output is compensated through pixel modulation of the sub-channels, thereby representing high grayscale information.
However, when the screen dithering method is applied, the same 2×2 pattern is repeated, and the output may have the same repeated pattern. Accordingly, a regular pattern may occur.